The code requires that an int is at least 32 bit wide.
If that is not the case and sizeof(int) is less than 32 bit you would get an error like:

error: size of array 'require_32_bit_integer_at_least' is negative

The line

(void) require_32_bit_integer_at_least; // no warning please

seems to be to avoid the "unused variable" warning. However, as the array is not defined as static the compiler would not generate a warning anyhow.
Knowing that a global non-static variable is really unused would require checking all translation units (source files) in the entire project.

As proposed by @MartinBonner, to make it clearer and easier to use, we could define a "static assert" macro: